A color cathode ray tube generally employs a black matrix-type phosphor screen in which a light-absorption layer (so-called black matrix) is formed among red, green and blue phosphor stripes (or dots) in order to improve the contrast thereof.
The black matrix-type phosphor screen is generally formed as follows.
As shown in FIG. 1A, the inner surface of a cathode ray tube panel 1 is first rinsed by an aqueous hydrofluoric acid solution. Then, a polyvinyl alcohol type photosensitive film 2, made by adding ammonium dichromate to polyvinyl alcohol, is coated on the inner surface of the cathode ray tube panel 1 and then dried.
As shown in FIG. 1B, a color selecting mechanism, for example, an aperture grill 3 is then used as an optical mask to expose the photosensitive film 2 to ultraviolet rays. In this exposing process, the photosensitive film 2 is exposed three times in the sequential order of, for example, green, blue and red colors. In FIG. 1B, reference numerals 4R, 4G and 4B designate exposing light sources corresponding to the red, green and blue colors, respectively.
Thereafter, the photosensitive film 2 is developed by some suitable process such as rinsing with water or the like to obtain a resist layer, in this example, a resist layer 5 (5R, 5G, 5B) of stripe-configurations on the inner surface of the cathode ray tube panel 1 at positions corresponding to the respective red, green and blue colors as shown in FIG. 1C.
After developing the film 2 with water, boric acid is dispensed onto the panel including the resist layer 5, producing very straight edges on the PVA and thus eventually to the carbon stripes (see FIG. 1D). Then, they are dried. This process will be hereinafter simply referred to as a hardening-process. In the hardening-process, a hardening agent used thereon may be generally a boric acid aqueous solution, or other aqueous solutions such as an aqueous tannic acid solution.
Then, as shown in FIG. 1E, a carbon slurry 6 is coated on the whole face of the inner surface including the resist layers 5R, 5G and 5B, and then dried. At the next step, water 7 is sprayed to uniformly wet the carbon-coated surface as shown in FIG. 1F. This wetting-process is carried out in order to provide a buffer against the extremely strong oxidant which, upon application, soaks through the carbon layer and dissolves the underlying PVA by breaking up the polymerized chains, thus also removing any carbon overlying it.
Then, the resist layer 5 is swollen and the developing-treatment is performed by means of, for example, water-spray, to remove the resist layer 5 and the carbon-coated layer formed on the resist layer 5, thus carbon-stripes or black matrixes 8 of predetermined pattern are formed on the inner surface of the cathode ray tube panel 1 as shown in FIG. 1G.
Thereafter, a phosphor slurry of, for example, green color is coated thereon and dried. Then, the coated green phosphor slurry is exposed by means of the aperture grill 3 (see FIG. 1B) and developed so as to form a green phosphor stripe 9G on the inner surface of the cathode ray tube panel 1 at its predetermined portion where the black matrix 8 is not formed. In a like manner, a blue phosphor stripe 9B and a red phosphor stripe 9R are formed on the inner surface of the cathode ray tube panel 1 at its predetermined portions, thus a color phosphor screen 10 of black matrix-type is formed as shown in FIG. 1H.
In the above-mentioned carbon coating process, though not shown in the figures, a carbon slurry from a carbon slurry tank is supplied through a supplying-system pipe to carbon-coating means, i.e., a nozzle. The carbon slurry is then ejected from the nozzle onto the inner surface of the cathode ray tube panel. After the carbon-coating film is uniformly coated on the inner surface of the cathode ray tube panel, the excess carbon slurry is collected back into the carbon slurry tank through a collection-system pipe.
Generally, when the carbon slurry used in this coating process is collected and continuously recycled, the viscosity of the carbon slurry increases. At a certain point in time, the carbon slurry becomes unsuitable for use in the coating process because the carbon-coated film does not dry well after application to the Panel 1 because of the increase in the viscosity of the carbon slurry. For example, if the viscosity of the carbon slurry is increased as high as four times its original viscosity, the thickness of the carbon-coated film is increased three times, making the carbon-coated film increasingly difficult to dry because it does not dry evenly and sufficiently. When the carbon-coated film that should be left in the afore-mentioned developing-process is too viscous, there is undesirable peeling. This phenomenon occurs especially on the peripheral portions of the panel.
In the existing process, in order to control the viscosity of the carbon slurry so that it always falls in a constant range, each time the carbon slurry is coated on a predetermined number of panels, the carbon slurry exhaust port of the carbon slurry tank is opened to dump some of the slurry to allow a predetermined liquid level to remain in the tank. Thereafter, fresh carbon slurry is added to the carbon slurry tank up to its predetermined liquid level. This purging technique increases the amount of carbon slurry consumed. The manufacturing cost of the black matrix of the color cathode ray tube is inevitably increased.